feat: add external MCP server support with streamable-http proxy

Support non-containerized MCP servers via externalUrl field and add
streamable-http session management for HA MCP proof of concept.

- Add externalUrl, command, containerPort fields to McpServer schema
- Skip Docker orchestration for external servers (virtual instances)
- Implement streamable-http proxy with Mcp-Session-Id session management
- Parse SSE-framed responses from streamable-http endpoints
- Add command passthrough to Docker container creation
- Create HA MCP example manifest (examples/ha-mcp.yaml)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/db/prisma/schema.prisma

@@ -57,6 +57,9 @@ model McpServer {
   dockerImage   String?
   transport     Transport       @default(STDIO)
   repositoryUrl String?
+  externalUrl   String?
+  command       Json?
+  containerPort Int?
   envTemplate   Json            @default("[]")
   version       Int             @default(1)
   createdAt     DateTime        @default(now())